Set Twitter language for WordPress oEmbeds

gistfile1.php

if ( !function_exists( 'ucc_oembed_twitter_lang' ) ) {
function ucc_oembed_twitter_lang( $provider, $url, $args ) {
	if ( 'twitter.com' == parse_url( $url, PHP_URL_HOST ) ) {
		if ( defined( 'WPLANG' ) )
			$lang = strtolower( WPLANG );
		if ( empty( $lang ) )
			$lang = 'en';

		$args['lang'] = $lang;
		$provider = add_query_arg( 'lang', urlencode( $lang ), $provider );
	}
	return $provider;
} }
add_filter( 'oembed_fetch_url', 'ucc_oembed_twitter_lang', 10, 3 );

/*
	Copyright 2012 Jennifer M. Dodd (email: [email protected])
	Released under the GPLv2 (or later).
*/

Here proper fix:

add_filter( 'oembed_fetch_url', 'firmasite_oembed_twitter_lang', 10, 3 );
function firmasite_oembed_twitter_lang( $provider, $url, $args ) {
    if ( 'twitter.com' == parse_url( $url, PHP_URL_HOST ) ) {
        $lang = explode("-", get_bloginfo('language'));
        $provider = add_query_arg( 'lang', urlencode( $lang[0] ), $provider );
    }
    return $provider;
}

Details: http://unsalkorkmaz.com/twitter-embeds-in-wrong-language/